The present invention relates to an exposure method and apparatus. More specifically, the present invention relates to an exposure method and apparatus which controls illumination intensity.
In recent years, liquid crystal display devices have been used as display elements for word processors, personal computers, TV sets, etc.
A liquid crystal display device is formed by photolithographically patterning transparent thin-film electrodes on a glass substrate. An exposure apparatus is known as an apparatus to serve the purpose of photolithography. The primary pattern formed on a mask is exposed onto a photoresist layer formed on the glass substrate via a projection lens.
Recently, the trend toward liquid crystal display devices has become greater and an increase in the size of the glass substrate has been demanded. From a manufacturing efficiency point of view, a plurality of display devices are exposed at the same time in many cases. Also an increase in the exposure region for an exposure apparatus has been required.
To meet the demand for larger exposure regions, a scanning exposure apparatus having a plurality of projection lenses as shown in FIG. 1 has become available.
In the prior art apparatus of FIG. 1, a mask 2 has a photolithographic pattern printed thereon. A plate 4 (glass substrate) has an upper photoresist surface and is attached to a carriage (not illustrated). Between the mask 2 and the plate 4 are located a plurality of optical systems or projection lenses 5 through 9 which direct a set of divided erect images of a pattern on the mask 2 to the plate 4. The pattern from the mask 2 are thereby printed on the plate 4 in a piecewise manner.
As illustrated, the projection optical systems 5, 6, 7, and 8, 9 are spaced at a predetermined distance from each other and each of the exposure regions are staggered such that they slightly overlap each other.
An illumination system 3 for exposure comprises illumination units 3a through 3e whose optical axes are respectively aligned with the projection lenses 5 through 9. An illumination adjustment mechanism (not illustrated) is built into the illumination units 3a through 3e.
In the above prior art configuration, after the mask 2 and the plate 4 are aligned, the relative position and distance between them are maintained while they are moved in the scanning direction A with respect to the illumination system 3 and the projection lenses 5 through 9. The mask 2 and the plate 4 are moved by a carriage drive mechanism such that the photomask patterns which continue on the mask 2 are divided by means of the projection lenses 5 through 9 to be exposed onto the plate 4.
In this way, the pattern on the entire surface of the mask 2 is transferred onto the plate 4. However, when the illumination intensities are dispersed among each of the projection lenses, the resulting re-synthesized pattern has various line widths which are dependent on the arrangement of the projection lenses.
Luminous intensity is dispersed due to the variation in the transmittivity of the projection optical systems 5 through 9 and the variation in the illumination intensity of the illumination units 3a through 3e. In order to prevent such unfavorable conditions, a new configuration having a sensor S0 for measuring light has been proposed so as to control the illumination intensity based on the output from the illumination sensor S0.
The sensor S0 for measuring light is installed onto a carriage (not illustrated) holding the mask 2 and the plate 4 such that the sensor is movable in the B direction of FIG. 1 on the carriage.
The carriage is also movable in the A direction for scanning exposure on the mask 2 and the plate 4. If the projection lenses 5 through 9 are relatively movable to a point at the right edge of the mask 2 and a point at the right edge of the plate 4 in FIG. 1, the sensor S0 can measure illumination intensity at an arbitrary point for each of the projection lenses by scanning with the sensor S0 in the B direction from such point.
FIGS. 2 through 5 show how illumination is measured in such prior art arrangement.
FIG. 2 shows the positional relationship between the carriage and optical systems when measuring illumination for projection lenses 8 and 9.
The sensor S0 is moved in the B direction to measure illumination for the projection lenses 8 and 9.
FIG. 3 shows the shapes and arrangement, on the imaging surface of the plate 4, of the exposure regions of the projection lenses 5 through 9. Each of the projection lenses 5 through 9 has a built-in field stop which comprises a trapezoid exposure region shown in FIG. 3. Each stop is arranged such that the shorter edge of the trapezoid exposure region of a set of the projection lenses 5 through 7 faces the shorter edge of the stops for lenses 8 and 9. Viewing from the scanning direction A, it is clear that the set of projection lenses 5 through 7 facing the set of lenses 8 and 9 are arranged such that the slopes of the trapezoid exposure regions overlap each other.
In prior art apparatus, to serve the purpose of controlling illumination, a method of measuring the illumination of the overlapped portions of the exposure regions has been proposed. In other words, as shown in FIG. 3, illumination intensities I1, I1', I3, and I3' are measured for the overlapped portions a1 through a4 respectively.
As shown in FIGS. 4 and 5, the carriage can be moved so that the illumination intensities I0, I0', I2, I2', I4, and I4' are measured in the same manner for the overlapped portions b1 through b6, respectively, of the set of projection lenses 5, 6, and 7.
Examples of the results obtained from the illumination measurements performed with the above prior art method are shown in FIG. 6. If there is a difference in illumination intensities (I0'-I1, I1'-I2, I2'-I3, I3'-I4) between adjacent projection lenses, the width of the line in the photomask pattern changes sharply. Thus zero or minimum illumination differences can be made by means of illumination adjustment mechanisms built into the illumination units 3a through 3e.
FIG. 7 shows the result of an adjustment example in which illumination intensities of the projection lenses 6, 7, 8, 9 are adjusted such that the difference in illumination between their overlapped portions is zero while fixing the illumination intensity I0 of the projection lens 5. In other words, using the illumination of the projection lens 5 as a standard, each of the illumination system units 3a through 3e of the illumination system 3 is controlled according to the illumination intensity I0 such that the illumination intensities at overlapped portions conform as I0' and I1, I1' and I2, I2' and I3, and I3' and I4 for the projection lenses 8, 6, 9, 7 in that order to eliminate a sharp change in illumination intensities for each of the positions of the projection lens 5. J0 through J5 in FIG. 7 shows the final illumination at overlapped portions for each of the positions of the projection lens 5.
In the above conventional technique, a scanning mechanism is required for light measurement, making the configuration complex. This provides problems in terms of cost effectiveness. Also scanning by means of the illumination sensor is time consuming. In addition, because scanning for measuring light takes place outside the scanning exposure regions, the illumination difference cannot be checked during the exposure sequence. This provides a further problem.
Moreover, regarding the exposure control, in the technique in which the measurement taken by a sensor at the edge is used as a baseline, if the projection lens at the edge provides an unusual value, it may be impossible to obtain a normal exposure under certain circumstances. This is due to the fact that other illumination intensities are determined based on the measurement taken by the sensor.